The present invention relates to a defect inspection method, and in particular to a defect inspection method of semiconductor products that requires defect inspection of high precision.
As the semiconductor design rule becomes finer, the size of manufacturing defects of semiconductor products becomes extremely small. For observation of semiconductor defects, scanning electron microscopes (hereafter referred to as SEMs) have begun to be used besides conventional optical microscopes. However, secondary electrons typically detected in SEMs have a problem that images with edges emphasized are often picked up and defects cannot be necessarily actualized favorably.
Therefore, a technique of detecting reflected electrons together with secondary electrons are detected and detecting a defect by using both detected signals complementarily has begun to be applied. Since reflected electrons emitted from the inspection subject has a directivity, an output correlated with an inclination of a three-dimensional slope in a position of electron beam irradiation is obtained and it can also be utilized for obtaining a defect shape.
It is known that the method utilizing reflected electrons is effective in detecting defects. For example, in a technique disclosed in U.S. Pat. No. 5,659,172, defects are extracted by detecting perspective images from a plurality of different directions in each of inspection positions and reference positions corresponding to the inspection positions, comparing perspective images detected from the same direction in an inspection position and a reference position, creating a plurality of comparison maps, and conducting computation on comparison maps.
However, the conventional technique has a problem that comparison maps created by comparing perspective views are susceptible to noise, and consequently minute defects cannot be detected stably. As the method for detecting perspective images, a technique of detecting only electrons emitted from a subject into a certain narrow angle direction by applying an electron beam to an imaging subject is typical.
As compared with the case where the angle is not restricted, the electron detection intensity becomes small, and consequently the signal-to-noise ratio of the perspective views is aggravated. When comparing perspective images picked up in a defect position and a reference position at high magnification, local perturbation is usually applied in many cases in order to allow manufacturing tolerance of non-defective portions. It is now supposed that two images, i.e., an image 1 and an image 2 are compared with each other in local perturbation. When an evaluation pixel in the image 1 is compared with a reference pixel in the image 2 corresponding thereto, a region corresponding to the manufacturing tolerance around the reference pixel in the image 2 is set. A difference between the evaluation pixel value and a pixel value that is closest to the evaluation pixel value among all pixels in the set region is outputted as a difference in the inspection pixel between the image 1 and the image 2.
This algorithm has a problem that when the signal-to-noise ratios of compared images are poor, a signal component of an obtained output image remarkably lowers. Because of this problem, there is a tendency to overlook minute defects in a high magnification state.